Bulk (i.e. mass) free-radical polymerization of pure monomer typically involves high heat of reaction (i.e. highly exothermic), increasing solution viscosity as polymerization progresses and the corresponding decrease in heat transfer coefficient of the reacting material. Because of these problems, controlling the temperature of bulk polymerization processes can be extremely difficult. However, it is well known to those skilled in the art that maintaining the desired temperature, is very important because of the strong dependence of the free-radical reaction kinetics on the reaction temperature, directly affecting the polymer properties such as molecular weight distribution and molecular weight. If the heat released from reaction exceeds the heat removal capability due to decreasing heat transfer, uncontrolled runaway can result where the rate of reaction increases as the temperature escalates due to exothermic reaction.
To circumvent these problems, free-radical solution polymerization is commonly performed where a non-reactive solvent in which the monomer and polymer are both soluble is used to reduce the heat load as well as to increase the heat transfer coefficient of the reacting mixture to facilitate temperature control. Alternatively, the heat load and viscosity/heat transfer problems are commonly managed by suspension polymerization and emulsion polymerization approaches. Solution polymerization, suspension polymerization, and emulsion polymerization approaches are disadvantageous in that they require extra equipment arid extra processing. Solution, suspension, and emulsion polymerization provide a decreased yield over bulk polymerization for a specific reactor volume. Emulsion and suspension polymerization offer the possibility of contaminants being introduced into the polymer from the surfactants and/or emulsifiers used in the polymerization process. Contaminants can also be introduced through impurities in the solvent in solution polymerization. Further, in the case of solution polymerization, solvent handling can be dangerous because of the threat of fire and/or explosion. Solvent handling can be expensive because extra equipment may be necessary to capture the solvent for reuse or other capture method, such as thermal oxidizers, may be required to prevent the compounds from being vented to the atmosphere.
Bulk free-radical polymerization heat transfer difficulties can be often managed in continuous processes. For example, reactive extrusion has been disclosed (U.S. Pat. Nos. 4,619,979; 4,843,134; and 3,234,303) as a useful bulk polymerization process because of the high heat transfer capability due to the large heat transfer area per unit reacting volume and the extremely high miring capability. Similarly, a continuous static mixer reactor with high heat transfer area for temperature controlled bulk free radical polymerization has been disclosed in U.S. Pat. No. 4,275,177.
As a rule, runaway free-radical polymerization reactions are not practiced because of their potentially disastrous consequences (Principles of Polymerization, Odian, G., 3rd Edition, Wiley-Interscience, p. 301, 1991). Generally, methods are used to control batch bulk polymerization reaction temperature to prevent runaway (i.e., U.S. Pat. Nos. 4,220,744, 5,252,662, JP 56185709).
Biesenberger et al. investigate batch runaway polymerization ("A Study of Chain Addition Polymerizations with Temperature Variations: I. Thermal Drift and Its Effect on Polymer Properties," J. A. Biesenberger and R. Capinpin, Polymer Engineering and Science, November, 1974, Vol. 14, No. 11, "A Study of Chain Addition Polymerizations with Temperature Variations: II. Thermal Runaway and Instability--A Computer Study," J. A. Biesenberger, R. Capinpin, and J. C. Yang, Polymer Engineering and Science, February, 1976, Vol. 16, No. 2, "A Study of Chain Addition Polymerizations with Temperature Variations: III Thermal Runaway and Instability in Styrene Polymerization--An Experimental Study," D. H. Sebastian and J. A. Biesenberger, Polymer Engineering and Science, February, 1976, Vol. 16, No. 2, "A Study of Chain-Addition Polymerizations with Temperature Variations. IV. Copolymerizations--Experiments with Styrene-Acrylonitrile," D. H. Sebastian and J. A Biesenberger, Polymer Engineering and Science, February, 1979, Vol. 19, No. 3, "Thermal Ignition Phenomena in Chain Addition Polymerizations," J. A. Biesenberger, R. Capinpin, and D. Sebastian, Applied Polymer Symposium, No. 26, 211-236, John Wiley & Sons, 1975). In Part II of the Biesenberger et al series, potential benefits of runaway polymerization are suggested. However, the purpose of the series is to understand runaway polymerization in order to prevent it. The series does not teach practical aspects of useful runaway polymerization in an industrial setting, as disclosed in the present invention. Adiabatic conditions are not employed in the Biesenberger et al. runaway polymerizations.
Continuous free-radical polymerization processes have been disclosed which involve adiabatic polymerization in tubular reactors (U.S. Pat. No. 3,821,330, DE 4235785A1). These approaches use equipment more complicated than a batch reactor.
Although industrially important, batch (non-continuous) reactors are less frequently used for bulk free-radical polymerization. The prime difficulty with batch reactors is that the heat transfer area per unit reacting volume is poor and becomes increasingly poor with larger reactor size. Methods of free-radical polymerization for acrylate pressure sensitive adhesive (PSA) production in batch reactors have been disclosed where polymerization chemistry is adjusted to slow the reaction rate so that the reaction temperature can be controlled (U.S. Pat. No. 5,252,662, JP 56185709). The difficulty with these approaches is that the heat transfer area of the batch reactor is still being relied upon to control reaction temperature by removing the heat of reaction and prevent runaway. Therefore, these polymerization approaches will not scale up directly because of the varying heat transfer capability with batch reactor size and they will be difficult to perform in large batch reaction equipment because of the increasingly poor heat transfer per unit volume with reactor size. Further, in controlling the heat load by slowing the reaction rate, the cycle time and thus productivity of a reaction vessel is decreased.
Batch reactors are desirable over continuous reactors in certain instances. For example, a specialty chemical manufacturer tends to produce multiple products. In this case batch reactors can be beneficial because of their multipurpose nature (i.e. not necessarily designed for a particular product or chemistry as is often the case with continuous equipment). In addition, often the economics of a batch reactor are favorable over that for a continuous process because of the relative simplicity of a batch reactor equipment. Typically, continuous processes become economical for high-volume commodity products (i.e. polystyrene).
In addition, the use of batch reactors for adhesive production is common because of the economics of their typical production volumes. Common monomers that are a major contributor to the composition of pressure sensitive adhesives (see below) have relatively high boiling points, and because of their relatively high molecular weights, have relatively low heat of reaction per unit mass. Therefore, the adiabatic temperature rise is such that the resulting mixture vapor pressure during reaction remains below about 100-300 psig (792.9-2171.8 kPa), pressures handled by common batch reactor equipment.
Advantages of bulk polymerization to produce hot-melt adhesives over other conventional polymerization methods are described in U.S. Pat. No. 4,619,979.